1. Field of the Invention
This invention relates to a computer aided manufacturing (CAM) process computer program that generates commands for use by a computer numerical control (CNC) milling machine, and more particularly, a computer program that generates commands for a CNC milling machine for forming a starting hole in a workpiece.
2. Background Information
The milling of an inside pocket in a surface of a workpiece by a vertical milling machine generally requires that a starting hole of preferably small diameter be first cut. The starting hole is cut using a material entry tool path which moves a cylindrical cutting tool in an axial direction, approximately perpendicular to the surface of the workpiece, from outside the workpiece, to a predetermined depth below the surface of the workpiece. After the cutting tool reaches the predetermined depth, a constant depth machining process in the horizontal plane typically commences for milling the pocket.
Two methods are commonly used to cut the starting hole. The first method uses a drill bit as the cutting tool. The material entry tool path is in the direction of the axis of the drill bit. Following the forming of the starting hole, a milling cutter of smaller diameter than the starting hole is fed into the starting hole in order to start the milling in the horizontal plane. Disadvantageously, drill bits suitable for drilling in metal have pointed ends, thus requiring the starting hole to be cleaned out by an extra milling operation before constant depth milling in the horizontal plane can commence. A further disadvantage of using a drill bit for a starting hole is that the drill bit must be exchanged for a milling cutter before horizontal milling for cutting a pocket can be initiated.
A second, and more recent method of forming a starting hole, uses an end mill. An end mill typically includes on its side, two or more helical shaped blades, called teeth, which have sharp cutting edges. The blades or teeth are separated by recessed helical grooves called flutes. Typically, the bottom or end of the end mill also contains two or more blades or teeth with sharp cutting edges. Consequently, an end mill can cut material on it's sides, by moving in horizontal direction relative to axis of the end mill, or can cut material on it's end, by moving in a direction of the axis of the end mill. With simultaneous motion in horizontal and axial directions, an end mill cuts with portions of both its side and bottom cutting edges.
Forming a starting hole with an end mill generally involves a material entry tool path having a helix-like shape, i.e. a helix whose radii may vary as a function of angle and/or depth, that advances into the workpiece in a direction generally perpendicular to the surface of the workpiece. If the helix is circular of constant radius, as is usual in the prior art, a single hole having a constant radius that is equal to the radius of the helix plus the radius of the cutting tool is formed to the desired depth of the first horizontal milling operation. By using an end mill in a helix-like tool path to form the starting hole, the bottom of the starting hole is flat and the milling cutter need not be changed to initiate the horizontal milling process for cutting the pocket.
The use of an end mill in a material entry toolpath is not without problems. During a machining process with an end mill, thin chips of material are cut, sheared or shorn from the workpiece. If the chips are not sufficiently evacuated from the work piece during the cutting process, they interfere with the cutting action of the tool by jamming between the cutting edges of the milling cutter and the work piece. The problem is exacerbated during the forming of starting hole because the preferably small diameter of the starting hole provides only a limited space for the chips to move away from the tool. While at shallow depths, the chips created by a helix-like tool path can escape relatively easily through the open space along the flutes of the milling cutter. However, as the depth of cut increases, friction between the chips, the walls of the work piece and the flutes, increases to a point where chips start to be packed together. Consequently, it becomes increasingly more difficult for the accumulation of chips to be evacuated through the space between the flutes as the depth of the starting hole is made larger. Beyond a certain depth of cut, some chips get caught and squeezed between the cutting edges of the tool and the wall of the region from which material has already been removed. At some point, the chips can become so tightly packed that the milling cutter is forced to re-cut them. This re-cutting interferes with the normal cutting action of the end mill in that some chips actually wedge between the cutting edge of the milling cutter and the work piece. This wedging action damages the milling cutter's cutting edge by causing small chips in the coating of a carbide milling cutter. Too many such small chips in the cutting edge can cause tool failure and possible breakage of the milling cutter. In softer materials, such as aluminum, the friction can cause the squeezed chips to rapidly heat to the melting point and become welded to the milling cutter, completely compromising its cutting ability and leading to failure or breakage of the milling cutter.
Prior art CAM software typically mills pockets by a succession of shallow horizontal milling operations in which the depth of each cut is less than one-half the diameter of the milling cutter. Such shallow cutting depths generally avoid the problem of chip re-cutting. The disadvantage of this method is that several cuts at shallow depths are required to get down to the desired depth of the pocket.
Recent advances in CAM software for controlling CNC milling machines, such as Truemill®, made by Surfware, Inc., allows the depth of each horizontal milling cut for forming a pocket to be at least as great as twice the diameter of the milling cutter, thus increasing the removal rate of material. However, forming a starting hole with a milling cutter to such depths using known material entry tool paths frequently results in excessive tool wear and possible tool breakage.
In consideration of the above, it would be desirable to have a material entry tool path that would allow for machining a starting hole having a depth of at least twice the diameter of a milling cutter and still avoid the problem of having chips of material accumulating between the cutting edges of the tool and the walls of the region from which material has already been removed.